title: Direct measurement of osmotic pressure via adaptive confinement of quasi hard disc colloids.
abstract: Confining a system in a small volume profoundly alters its behaviour. Hitherto, attention has focused on static confinement where the confining wall is fixed such as in porous media. However, adaptive confinement where the wall responds to the interior has clear relevance in biological systems. Here we investigate this phenomenon with a colloidal system of quasi hard discs confined by a ring of particles trapped in holographic optical tweezers, which form a flexible elastic wall. This elasticity leads to quasi-isobaric conditions within the confined region. By measuring the displacement of the tweezed particles, we obtain the radial osmotic pressure. We further find a novel bistable state of a hexagonal structure and concentrically layered fluid mimicking the shape of the confinement. The hexagonal configurations are found at lower pressure than those of the fluid, thus the bistability is driven by the higher entropy of disordered arrangements, unlike bulk hard systems.
authors.affiliation: 1] H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK [2] School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK [3] Centre for Nanoscience and Quantum Information, Tyndall Avenue, Bristol BS8 1FD, UK.
authors.forename: Ian
authors.initials: I
authors.lastname: Williams
authors.forename: Erdal C
authors.initials: EC
authors.lastname: Oğuz
authors.forename: Paul
authors.initials: P
authors.lastname: Bartlett
authors.forename: Hartmut
authors.initials: H
authors.lastname: Löwen
authors.forename: C Patrick
authors.initials: CP
authors.lastname: Royall
mesh_terms: D002623:Chemistry Techniques, Analytical; D003102:Colloids; D003198:Computer Simulation; D004548:Elasticity; D019277:Entropy; D000431:Ethanol / Q000737:chemistry*; D008956:Models, Chemical; D009010:Monte Carlo Method; D052898:Optical Tweezers; D009997:Osmotic Pressure; D011137:Polystyrenes / Q000737:chemistry*; D016062:Porosity; D011312:Pressure; D014867:Water / Q000737:chemistry*
chemical_list: D003102:Colloids; D011137:Polystyrenes; D014867:Water; D000431:Ethanol